1. Field of the Invention
The invention relates to the use of electrolytes for controlling the flow properties of emulsions of the oil-in-water (O/W) type.
2. Description of the Related Art
Emulsions as oil-in-water (O/W) dispersions are widely used as an application form for coating materials, such as, for example, water-based paints and finishes, as adhesives and sealants, such as, for example, aqueous epoxy or polyurethane systems, as cosmetic formulations, as cleansing agents and disinfectants, in the food industry, for the surface modification of solid substrates or as reaction media in emulsion polymerization.
In general, the dispersing and stabilization of the disperse phase are effected with the aid of emulsifiers. Cationic, anionic, ampholytic and nonionic emulsifiers are used. Common to the emulsifiers is that they are surface-active substances. That is to say, they preferably accumulate at interfaces, such as, for example, liquid-liquid, liquid-solid or liquid-gas interfaces, and thus reduce the interfacial/surface energy. On application of the emulsion, however, the emulsifiers can also cover the surface of the substrate to be treated and thus greatly change the wetting properties of the surface. This can adversely affect, for example, the adhesion properties of a coating material or of an adhesive joint or seal. Furthermore, the recoatability may be adversely affected. In addition, emulsifiers based on organic molecules are potential hazardous substances when used in pharmaceutical or cosmetic formulations or in foods.
In 1907, Pickering described for the first time the preparation of emulsions which were stabilized only by addition of various solids, such as basic copper sulfates, basic iron sulfates or other metal salts. These types of emulsions are also referred to as “Pickering emulsions”. Basic investigations showed that a characteristic of Pickering emulsions is that solid particles are arranged at the interface between the two liquid phases and form a barrier there to the coalescence of the disperse phase.
In this case, it was shown that although such solid-stabilized emulsions, as were described, for example, in DE 198 42 787, are markedly stable against coalescence of the disperse (oil) phase, they have high viscosities, in particular at phase volumes Φ of the disperse phase of greater than or equal to 0.5, which can have negative effects on the application properties of the emulsions. Attempts at achieving low viscosities by reducing the phase volume of the disperse phase or through lower use amounts of particulate emulsifier often lead to unsatisfactory separation-stable emulsions.
Precise setting of the viscosity according to the requirements of the relevant application is imperative for successful use of emulsions, for example as coating material.